The prime objective of the proposed experiments is to optimize the immunogenicity of the HIV env protein when expressed in vivo from a recombinant alphavirus expression vector. A full-length CDNA clone of a member of the alphavirus genus, the Venezuelan equine encephalitis virus (VEE), has been modified to generate a live attenuated vaccine candidate for humans. It is anticipated that this new, molecularly-derived vaccine strain will replace the currently available VEE vaccine strain TC83. VEE causes an acute infection in which the initial phase is characterized by replication in dendritic cells in the draining lymph node. The vaccine strain carries out this initial phase without causing disease, and has been modified to be either a propagation-competent expression vector or a propagation-defective vector (replicon) capable of RNA amplification and expression in single infected cells. In a model system, expression of the influenza virus HA protein using these vectors induced immunity in mice that afforded complete protection against a lethal mucosal challenge of influenza virus, and that gave significant protection of the respiratory epithelium against infection by the challenge virus. The applicants have used this vector to express the HIV-1 MA/CA coding domains at high levels in cultured cells and to induce humoral, mucosal, and cellular immune responses to gag in mice. They also have expressed HIV-1 env using these vectors, but the levels of protein expression are significantly lower compared to that obtained with the MA/CA coding domains. In addition, the immune response to HIV-1 env is significantly lower than to influenza virus HA using the same vectors. Furthermore, the applicants will attempt to introduce a series of modifications in the HIV-1 env gene to express the env protein in new ways within the context of the promising VEE vaccine vector in an attempt to express this protein at high levels and in a form that will enhance its intrinsic antigenicity. The effect of these modifications will be monitored by examining the level of expression of the env protein in cell culture and by assessing the ability of the env-expressing vector to induce neutralizing antibody in vivo.